Similar to muffin top , which means fat bulging out of ones hips from pants that are too tight, muffin tin means that fat bulging out of multiple articles of clothing(i.e. cleavage that bulges over the seam of the shirt, and the sleeves of a tight shirt causing the arms to look bigger than they actually are)

"HA! That bitch thinks she's so hot, with all her tight clothes"
"Muffin top much?"
"Nah trick. More like muffin TIN"